Accused al Qaeda propagandist Ali Hamza al Bahlul waves a boycott sign at his May 7, 2008 arraignment on war crimes charges at Camp Justice, the military commissions complex, at the U.S. Navy base in Guantánamo Bay, Cuba, as shown in a courtroom sketch submitted to U.S. military scrutiny before release. At his right is a Pentagon paid translator. POOL SKETCH ARTIST

Accused al Qaeda propagandist Ali Hamza al Bahlul waves a boycott sign at his May 7, 2008 arraignment on war crimes charges at Camp Justice, the military commissions complex, at the U.S. Navy base in Guantánamo Bay, Cuba, as shown in a courtroom sketch submitted to U.S. military scrutiny before release. At his right is a Pentagon paid translator. POOL SKETCH ARTIST

A civilian appeals court on Monday vacated two convictions of a former aide to terrorist mastermind Osama bin Laden who is the only detainee at Guantánamo serving a life sentence.

In a split decision, the U.S. Court of Appeals for the District of Columbia Circuit concluded a military commission lacked authority when it convicted Ali Hamza al Bahlul of two out of three charges in November 2008.

It did not overturn a conviction on conspiracy to commit war crimes for serving as as bin Laden’s media secretary but vacated Bahalul’s convictions of, providing material support for terrorism and solicitation of others to commit war crimes.

Bahlul, who was known to shout “boycott, boycott, boycott” during Guantánamo court proceedings, mounted no defense at his trial. Pentagon prosecutors called him a propagandist, and trusted member of bin Laden’s inner circle.

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He admitted to creating an at-times cartoonish laptop computer recruiting video for the terror group al-Qaida. It glorified the October 2000 attack on the USS Cole warship off Yemen in which 17 U.S. sailors died.

The jury then sentenced him to life imprisonment.

In the 150-page decision Monday, the appellate court rejected Bahlul’s challenge to the conspiracy charge, but vacated the other two.

“Solicitation of others to commit war crimes is plainly not an offense traditionally triable by military commission,” Judge Karen LeCraft Henderson wrote, adding that “the government offers little domestic precedent to support the notion that material support or a sufficiently analogous offense has historically been triable by military commission.”

Bahlul‘s attorneys had argued that the Guantánamo war court could only consider actions that were recognized under the international law of war when committed. As part of its complex ruling, the appellate court did not agree, stating that the 2006 law establishing military commissions “is unambiguous in its intent to authorize retroactive prosecution for the crimes enumerated in the statute — regardless of their pre-existing law-of-war status.”

Monday’s ruling included three lengthy concurring-and-dissenting opinions, as well as an unusual separate opinion in which Henderson explained how she might have ruled otherwise but for a legal concession made by the Justice Department.

Bahlul is a native of Yemen. In the late 1990s, he traveled to Afghanistan to join al-Qaida.

He was captured in Pakistan three months after the Sept. 11, 2001 attacks, and was transferred to the United States Navy Base at Guantánamo Bay, Cuba, on Jan. 11, 2002, the day the Bush administration opened the detention center.

He is the lone Guantánamo prisoner imprisoned on a cellblock — restricted to convicts — in the detention center’s maximum-security Camp 5 prison because, although two other men have pleaded guilty to war crimes, they have yet to be sentenced.

Bahlul, the appellate court reported, “flatly refused to participate in the military commission proceedings and instructed his trial counsel not to present a substantive defense.”

The appellate court’s 4-3 decision could result in a revised sentence for Bahlul.